Densification mechanisms of tight sandstones in closed to semi-closed systems: typical example from the Upper Paleozoic in the Linxing area, Ordos Basin

The upper Paleozoic tight sandstone in the Linxing area on the eastern margin of the Ordos Basin is characterized by a wide planar distribution and multiple vertical layers that form a region of multi-layered tight sandstone natural gas reservoirs with strong heterogeneity. In this study, physical properties deciphered by using numerous core sample imaging techniques were used to determine the characteristics and formation mechanisms of the tight sandstone reservoirs. We conclude that there are four types of tight sandstone reservoirs in the upper Paleozoic strata of the study area and that the diagenetic evolution of the sandstone can be subdivided into three stages: an early compaction stage, characterized by severe porosity reduction; a middle stage, involving quasi-continuous dissolution–cementation via migrating organic acids generated by coal-measure source rocks; and a late stage, involving oil and gas charging and clay mineral conversion. These processes caused the reservoirs to assume the densification characteristics of multi-stage calcareous, siliceous, and clay mineral cementation and infilling. Our comprehensive analysis shows that compaction and quasi-continuous dissolution and cementation are the main reasons for the formation of tight sandstone. Although organic acids promote the development of dissolution, it is also the fuse of sandstone densification. The matching relationship between sandstone densification and hydrocarbon accumulation is characterized by densification followed by accumulation.